This invention relates to a method for preparing starch derivatives by reacting starch and an anhydride or substituted dicarboxylic acid anhydride wherein the anhydride reagent is added to the starch slurry in pre-emulsified form and provides a faster, more efficient reaction.
The preparation of starch derivatives and especially hydrophobic derivatives such as starch alkyl or alkenyl succinates is well known and has been carried out for many years. U.S. Pat. No. 2,661,349 issued on Dec. 1, 1953 to C. Caldwell and O. Wurzburg describes such starch derivatives and the aqueous method in which they are prepared using a standard esterification reaction where the reagent and starch suspended in water are mixed under alkaline conditions. Other methods for preparing the starch derivatives are disclosed in the '349 patent including an organic suspension or dispersion method and a so called dry method using starch in the commercially dry form, i.e., starch having a moisture content of approximately 5 to 20%. The starch derivatives and method of preparation are further described in "Starch: Chemistry and Technology", Second Edition, edited by R. L. Whistler et al., 1988, pp. 341-343 and "Modified Starches: Properties and Uses", edited by O. Wurzburg, 1986, Chapter 9, pp.131-147.
The reaction of hydrophobic reagents such as octenyl succinic anhydride (OSA) in aqueous systems is limited by the solubility of such reagents, moderate reaction efficiencies and long reaction times. Reagents with longer side chains, i.e., dodecenyl, hexadecenyl, etc., have even lower solubility in water and are much harder to react under the known conditions and in fact in some cases will not react at all.
Recent U.S. Pat. No. 5,672,699 issued on Sep. 30, 1997 to R. Billmers et al. discloses a method for preparing hydrophobic starch derivatives having improved reaction efficiencies wherein the starch and anhydride reagent are predispersed or intimately contacted at low pH before being brought to alkaline reaction conditions.
While the methods described above are useful in providing hydrophobic starch derivatives, there still is the need for a method which will provide reduced reaction time and improved reaction efficiency.